1. Field of the Invention
The present invention relates to an image forming apparatus, and particularly to an image forming apparatus in which a photosensitive material is exposed based on digital image data and an image corresponding to the digital image data is formed.
2. Description of the Related Art
There presently exists an image forming apparatus in which an image is formed on a recording medium by effecting a main scan operation and a sub-scan operation while modulating, based on digital image data, spot-shaped light beams (hereinafter referred to as "spot-light") emitted from a semiconductor laser or a light emitting diode (LED). Further, as this type of image forming apparatus, there exists an apparatus in which during a scan-exposure operation, the intensity of spot-light is modulated in accordance with digital image data so as to vary the density of dots to be formed, and dots having a density corresponding to the digital image data are formed on a recording medium.
As an image forming apparatus in which a light emitting diode (LED) is used as a light emitting element, an apparatus has been proposed in which a photosensitive material is exposed to light by effecting a main scan operation such that a plurality of scanning lines is synchronously recorded by a plurality of light emitting diodes arranged along a sub-scan direction. In this image forming apparatus, a predetermined range on the photosensitive material along the sub-scan direction can be exposed at one main scan operation by effecting a main scan operation synchronously using the plurality of LEDs each forming a line (hereinafter referred to as "main scan line") recorded by dots along the main scan direction and arranged along the sub-scan direction. Accordingly, even when a high-quality image is formed which has a high density of main scan lines, there is no possibility that the number of times of main scan increases. As a result, an image can be formed efficiently and in a short time.
However, when the predetermined range on the photosensitive material is exposed, the sub-scan operation needs to be effected to obtain an amount of movement corresponding to the predetermined range. At this time, when the amount of movement caused by the sub-scan operation cannot be correctly controlled, the space between a lowermost scan line in a preceding main scan operation and an uppermost scan line in a succeeding main scan operation varies from the space between scan lines recorded synchronously. As a result, streaked (i.e., linear) uneven density along the main scan direction is caused. Particularly, when exposure processing is effected repeatedly at a relatively wide range, the linear density unevenness is formed at fixed spaces and a finished quality of an image is thereby damaged.